The present invention relates to improvements in an automatic document feeder used for a recording device or an image reading device of an electrophotographic copier.
An automatic document feeder is generally provided on a platen glass in an image exposure section of a copier. In an optical system movement type copier to which the automatic document feeder is provided, documents are conveyed as follows: documents are sent out from a document stack tray (stack section) by the automatic document feeder one by one; conveyed onto the platen glass by a conveyance belt; and after the documents have been exposed by an exposure lamp provided in the copier body, they are discharged onto a document discharge tray.
In a conventional automatic document feeder, the document feeding operation is carried out in the following manner: after a paper discharge sensor has detected that a document on the platen has been discharged, the following document is sent out from the stack section onto the platen glass, and stopped at a predetermined position. Therefore, it takes a long period of time to replace a document placed on the platen. Accordingly, a large amount of time is required for a copying operation, and the productivity is low.
In the case where an automatic document feeder is used, the copying productivity can be increased to 100% when a document is replaced within the time in which the optical scanning system is returned. In order to replace the document within the time in which the optical scanning system is returned, it is necessary to increase the document conveyance speed when the document is conveyed from a double feeding prevention device of the document stack section to a document stop position on the platen glass. However, in order to convey the document at high speed, the following various problems are caused, such as an increase in the motor dimensions, an increase in electrical power consumption, noises, abrasion of the conveyance belt, damage of the document, and deterioration of the document separation property.
A document conveyance device of the prior art is disclosed in Japanese Patent Application Open to Public Inspection No. 236136/1989 in which the aforementioned problems are solved and the document replacing time can be shortened without an increase in the document conveyance speed.
This document feeder is operated in the following manner: the first document is stopped at a predetermined position on the platen glass; the second document is stopped at a position adjacent to the predetermined position, wherein the position is not located on the platen glass; after the first document has been exposed, it is conveyed to a downstream position adjacent to the predetermined position, wherein the downstream position is located on the platen glass; at the same time, the second document is conveyed to the predetermined position and stopped; and the third document and documents after that are conveyed in the aforementioned manner.
However, it is difficult to apply the aforementioned document conveyance system to two-sided documents.
According to the prior art, a two-sided document is copied in the following manner: the two-sided document is conveyed on to the platen glass; the first side of the document is exposed; the document is reversed by an inversion conveyance means; the document is conveyed onto the platen glass and the second surface is exposed; after that, the document is reversed by the inversion conveyance means; and the document is discharged outside the apparatus in a collated condition. In the aforementioned automatic two-sided document feeder, the interval from the trailing edge of the first document to the leading edge of the second document is large, so that the document replacement time is disadvantageous.